One of the most significant developments in semiconductor technology in recent years has been the increased use and importance of compound semiconductors. Particularly significant are the group III-V compounds composed of elements of groups III and V of the periodic table such as gallium arsenide and indium phosphide. Such materials are used, for example, for making lasers, light-emitting diodes, microwave oscillators and light detectors. Also promising are the II-VI materials such as cadium sulfide that may be used for making light detectors and other devices.
Most commercial use of compound semiconductors requires the growth of large single-crystal ingots from which wafers can be cut for the subsequent fabrication of useful devices. One of the more promising methods for such crystal growth is the vertical gradient freeze (VGF) method, particularly the VGF method described in the W. A. Gault, U.S. Pat. No. 4,404,172, granted Sept. 13, 1983, and in the paper, "A Novel Application of the Vertical Gradient Freeze Method to the Growth of High Quality III-V Crystals," by W. A. Gault et al., Journal of Crystal Growth, Vol. 74, pp. 491-506, 1986, both of which are hereby incorporated herein by reference. According to this method, raw semiconductor material is placed in a vertically extending crucible including a small cylindrical seed well portion at its bottom end which snugly contains a monocrystalline seed crystal. Initially, the raw material and a portion of the seed crystal are melted. The power to the system is then reduced in such a manner that freezing proceeds vertically upwardly from the seed crystal, with the crystal structure of the grown ingot corresponding to that of the seed crystal.
While the VGF method seems to work better than other methods for reducing the density of dislocations and other defects in the ingot, the number of imperfections in such ingots still constitutes a problem. It has been known that these often results from stresses at the interface of the crystal with the crucible and that such stresses can be reduced by growing the crystal in the &lt;111&gt; crystallographic direction (or, more specifically, the &lt;111&gt;.sub.B direction). Most semiconductor processes, however, require that the wafers from which devices are made be oriented in the &lt;100&gt; crystallographic direction or in a direction which is close to that direction. With the wafers oriented in the &lt;100&gt; direction, cutting the wafers into chips along crystallographic planes naturally forms rectangular structures, whereas if the wafers were oriented in the &lt;111&gt; direction, there would be a tendency to form trapezoidal or triangular structures by such processing. It is therefore customary, after growth of the ingot, to cut wafers from it at an angle with respect to the ingot central axis of about 35.3.degree. so that the upper surfaces of such wafers lie in the appropriate crystallographic plane. Since the ingot is cylindrical, this leads to the formation of elliptically shaped wafers (the segments cut from ingots will be referred to herein as "wafers" for expediency, although such segments are sometimes known as "slices," the term "wafer" being sometimes used to refer to the slice after it has been ground and polished).
The technology of operating on semiconductor wafers to produce integrated circuit chips and other useful semiconductor devices derives from the much older silicon technology in which circular wafers are cut from cylindrical ingots. As a consequence, it is difficult to use elliptically shaped semiconductor wafers efficiently and a significant wastage of usable semiconductor wafer area inherently accompanies the use of elliptical wafers. This has long been understood as a penalty one pays for growing compound semiconductors in the &lt;111&gt; direction, and the belief has been that if the requirement for low defect density is sufficiently stringent, then that is a price that one must pay. Accordingly, there has been a long-felt need for a method for growing compound semiconductor ingots with a minimum of defects, but from which one can cut circular wafers.